Many processes have been developed for the recovery of soda ash from salts such as occur in the brines of Searles Lake and Owens Lake in California, and in the relatively pure solid trona deposits of Wyoming. The solid salt mixtures and brines usually include sodium carbonate, sodium bicarbonate, sodium sulfate, sodium chloride, potassium salts and boron salts, and double salts such as trona, burkeite and glaserite.
Known processes for the treatment of trona include the sodium sesquicarbonate process and the monohydrate process. The former includes the saturation of boiling mother liquor with trona, clarification of the solution, removal of organic impurities, crystallization of sodium sesquicarbonate and calcination to sodium carbonate. The latter includes the decomposition of trona by calcination followed by leaching, liquid-solid separation, removal of organics and evaporation above 100.degree. C. to obtain sodium carbonate monohydrate which is calcined to anhydrous sodium carbonate. Without additional elaborate processing, these processes cannot be used for the treatment of salts with high contents of impurity salts such as chlorides and sulfates.
Searles Lake and Owens Lake salts and brines have been treated in the past by processes which mainly include the manipulation of brines in the burkeite field of the carbonate-sulfate-chloride phase diagram. Such processes are disclosed, for example, in U.S. Pat. Nos. 1,836,426, 1,836,427, 1,853,275, 2,348,164 and 2,392,888. These processes are all carried out so that either burkeite or sodium sulfate is precipitated. A major disadvantage of these processes is the need for excessive processing to recover the sodium carbonate from the burkeite and to recover the sodium carbonate in a satisfactorily pure form. According to U.S. Pat. No. 1,618,834, Owens Lake brine, or similar brines, is saturated and carbonated at 30.degree.-45.degree. C. with an amount of carbon dioxide sufficient to cause the precipitation of sodium sesquicarbonate (trona), which is separated from mother liquor, washed and further treated for the recovery of soda ash. The addition of a larger than sufficient amount of carbon dioxide would cause precipitation of sodium bicarbonate which is undesirable because of problems with the separation and purification of the sodium bicarbonate. It has been known, as is stated, to precipitate bicarbonate and then mix precipitated bicarbonate with sodium carbonate solution so as to cause precipitation of sesquicarbonate. As is stated in U.S. Pat. No. 1,853,275, the carbonation processes require the need of burning limestone to generate the carbon dioxide. It is also stated in the same patent that the trona in the Owens Lake deposit does not lend itself to form the basis of an economical process for the manufacture of soda ash. It is also noted that the carbonation processes disclosed in U.S. Pat. No. 1,618,834 are directed solely to the treatment of brine and not of solid salt mixtures. Many other processes, which may include dissolution of solid salts, followed by evaporation, concentration and fractional crystallization to effect separation of salts and recovery of the desired salt specie(s) from salt solutions, are expensive because elaborate and expensive equipment and large amounts of energy are required. The energy requirements can be considerably reduced by using a non-convective solar pond for the crystallization and dehydration of sodium carbonate decahydrate to its monohydrate form. The crystallization and dehydration of sodium carbonate decahydrate is disclosed in U.S. Pat. No. 4,179,493, according to which a sodium carbonate solution is added to a non-convective solar pond, the solution exceeds its solubility in the upper layer of the pond, decahydrate crystallizes and the crystals settle to the bottom of the pond where they are dehydrated. The process is directed to the treatment of solutions containing a single salt and does not disclose the possibility of treating salt mixtures for the separation and purification of sodium carbonate from such mixtures.